Plywood panel joint



May 27, 1941.

A. A. LAHTI PLYWOOD PANEL JOINT- Filed Aug. 1, 1958 Patented May 27, 1941 PLYWOOD PANEL JOINT Anard A. Lahti, Tacoma, Wash, assignor to Lahti Incorporated, Tacoma, Wash., a corporation of Washington Application August 1, 1938, Serial No. 222,415

2 Claims.

This invention relates to the joint between two veneer panels and especially to that type of joint which is to be left exposed to view and not to be covered by a batten or other fastening and concealing means, and has for its objects to provide means for locking the edges of two panels together, thus preventing the edge of one panel warping or springing out of the plane of its adjacent panel; to provide a joint which requires no additional time for the erector to install and which, in fact, saves the time taken to fit and fasten the usual battens for such panels; which leaves a perfectly smooth surface across the joint without any special fitting or fastening for the parts; which permits the joint between two panels to be placed between the studding of awall; which provides a continuous smooth surface which may be covered with wall paper or other decorative material; which is continuously crossed on its rear or under side by a series of separated tongues or tenons extending from each panel under the other panel and each having a substantial part of its grain directed across the joint, thereby preventing the breaking away of the edges of the panels and prolonging the life of the joint; which provides a countersunk space at the joint wherein the woodfiller becomes anchored and over which the final finish may be applied, thus formin an invisible and indistinguishable joint in the wall; and which is cheap to manufacture, easy to apply, and which effectively locks the two panels together.

I attain these and other objects by the devices and arrangements illustrated in the accompanying drawing, in which- Fig. 1 is a face view of a joint between two panels, constructed in accordance with my invention; Fig. 2 is a similar rear view thereof; Fig. 3 is a section thereof; Fig. 4 is a section, drawn to a larger scale and showing the direction of the grain of the several layers crossing the.

joint; and Fig. 5 is a perspective view of the under or rear side of one tenon, showing the directions of its grains.

Similar numerals of reference refer to similar parts throughout the several views.

. It has long been a problem to provide some means of fastening the adjacent edges of two plywood panels so that the usual battens could be dispensed with and thus provide a uniform continuous surface for a wall, but no adequate solution has heretofore been proposed because it proved practically impossible to prevent one edge of the thin plywood panel raising above the level of the other panel. This fact has resulted in the introduction of various artificial products, made of wood fiber or other materials, to the detriment of the plywood industry, whereas plywood is in other respects a much more satisfactory wall covering besides being cheaper in first cost and in cost of erection. As a rule the plywood used is three-ply and about one-quarter inch in thickness, with the grain of the two outer layers running longitudinally of the piece while the grain of the center layer runs transversely thereof. Under these circumstances panels can in practice he made about four feet in width and of indefinite length. It will be understood that the outer layer or ply of the panel may be made of any desired decorative wood, but that it is very thin and, taken by itself, is very fragile along the edges because its grain runs parallel with the edge.

Referring to the drawing, the sheets A and B of plywood have their adjacent side edges cut away in the following described manner to form the joint, the face grain of each of said sheets running substantially parallel with said adjacent side edges. The line I is the original edge of the sheet B and the line 2 is the original edge of the sheet A. The upper or face layer 3 of each of the sheets A and B is cut back to form the new face edge 4 of the sheet. The distance I cut this edge 4 back from the original edge is, in practice, about equal to the thickness of the said sheets A and B. This step of cutting back the face edge of the panel is preferably performed by running the panel longitudinally past a rotating cutter, in slightly tipped position relatively to the axis of rotation in order to form the undercut surface of the edge 4 (Fig. 4), and this cutter is shaped to remove the part of the layer 3 outside of the line 4- and to form the curved top surface 5 ofthe ledge left after said step is completed. This cut-back portion may be conveniently of Substantially the depth of the face layer 3 but may vary therefrom very slightly without serious detriment. The new edge 4 is slightly undercut, as clearly shown in Fig. 4, to form with the corresponding edge of the other. panel a dovetailed space adapted to receive and anchor the glue or woodfiller applied to the surface, and the top surface 5 is curved to conform to the under surface of the hereinafter described mortises.

At the end of this step it is evident that the side edge of the panel being operated upon is provided, first, with a slightly undercut step at the new edge 4, and second, with a stub-nosed ledge. The curvature of the top of the ledge corresponds with the circular arc of the mortise cutters, hereinafter described, and is such that the The step of making the tenons and mortises is briefly described as follows:

The side edges of the several panels having thus been cut back, as above described, each panel is passed in a lateral direction over a series of rotating cutters mounted on a shaft parallel with the edge of the panel (in practice these cutters should be about two inches in diameter) which cut away alternate parts both of the said ledge and of the center layer 6 and the under layer 1 inwardly of the cut back edge 4. I prefer to have these cutters about three-quarters of an inch in width and to space them apart a corresponding distance, allowing, however, sufficient tolerance to permit the tenons thus formed to enter the mortises. The panel edge is passed over the said cutters until the said new edge 4 is over the center of the revolvingshaft on which the cutters are mounted, thus completely cutting away portions of the above mentioned ledge tomake a series of tenons 8 in the spaces between the cutters and a series of mortises 9, alternating with said series of tenons, extending inwardly of and under the new edge 4. As above stated, the tenons 8 are stub-nosed, while it is evident that the mortises 9 will be cut on the arc of the cutter; therefore, as clearly shown in Fig. 4, there is a portion of each mortise 9 which cannot be filled by the corresponding tenon 8. This provides a series of pockets at the back of the panel into which the fastening glue may exude. Since the curvature of the upper surfaces 5 of the tenons 8 corresponds with the curvature of the under surfaces of the mortises 9 and the width of the mortises corresponds with that of the tenons, and since all the mortises are cut on the same machine, it is evident that the tenons of any one panel will fit into the mortises of any other panel, and that the tenons of each panel engaging under the under surface of the mortises actively prevent either panel from any relative motion; and since these tenons are narrow and continue the full length of the panel it is evident that both panels are rigidly held in alinement throughout their entire length.

The step at the edge 4 is necessary in order to give a definite line of joint; otherwise, if the cutters were to cut as deep as the panel, both panels forming the joint would have feather edges which would be broken off with ease and thus spoil the joint, especially as the tenons of each panel are relatively narrow and the upper surface or ply is very thin.

Since the new face edge is only the depth of the outer or face layer, whose grain is substantially parallel with the edge, the central layer, whose grain runs across the panel, crosses under the joint from one side to the other, alternately, and thereby gives strength to the tenons to prevent their breaking away from the respective panels in handling and in practical use.

The panel joint thus formed therefore effectively locks the two panels together against any relative motion and each panel will prevent the warping of the other panel out of the common plane, leaving a perfectly smooth surface at the joint, which joint can have any suitable decorative covering which is applied to the whole wall (such as staining, painting or wall paper) and will not be apparent to the casual observer. Further, since the joint is made of plywood and the central layer of the plywood has its grain running across the joint in the several tenons, the

joint is substantially as strong as the other parts of the panels. This fact also permits that the joint need not be placed over the center of one of the supporting studs, as in the present practice, but may be placed between the studding if convenient, without substantial difference, thus further speeding up the erection of the wall. Also it is evident to one familiar with the use of plywood in wall erection, that when plywood is made with my improved joint edge, the time of erection will be materially reduced by the fact that when one panel is fastened in place, the next panel is slipped into place beside it, the tenons of each entering the mortises of the other, and no time need be spent in getting the two panels to form a good joint since they automatically do so as above described.

It is to be understood that changes may be made in the details of my invention as outlined in the above description without departing from the spirit thereof as stated in the appended claims.

Having, therefore, described my invention, what I claim and desire to secure by Letters Patent, is:

1. A joint for plywood panels each panel having an outer layer of its original side edge cut back to form a substantially straight edge of a depth approximately equal to the thickness of said outer layer, the outer layers of the panels meeting in a butt joint, additional layers of said panels extending beyond said straight edges to form tenons, each of said tenons being stub-ended and including throughout its length at least a substantial part of a layer of wood having its grain running perpendicularly to the associated face edge of the panel, and a plurality of mortises complementary to said tenons extending inwardly beneath each face edge.

2. A joint for plywood panels each panel having an outer layer of an original side edge out back to form a substantially straight edge of a depth approximately equal to the thickness of said outer layer, each of said straight edges lying at an acute angle relatively to the face of the panel, the outer layers of the panels meeting in a butt joint, additional layers of said panels extending beyond said straight edge to form stubended tenons, each of said tenons having a curved outer surface and of such thickness as to include the major portions of two of the layers of Wood contained in said panel, and a plurality of mortises complementary to said tenons extending inwardly beneath each straight edge to the- 

